1. Field of the Invention
This invention relates generally to sources of electromagnetic laser radiation and, in particular, to unipolar semiconductor quantum cascade (QC) lasers and fabrication thereof.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. The possibility of amplification of electromagnetic waves in a semiconductor superlattice structure, i.e., the possibility of semiconductor diode lasers, was predicted in a seminal paper by R. F. Kazarinov, et al., xe2x80x9cPossibility of the Amplification of Electromagnetic Waves in a Semiconductor with a Superlattice,xe2x80x9d Soviet Physics Semiconductors, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductor laser technology has continued to develop since this discovery.
There are a variety of types of semiconductor lasers. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers of various types may be electrically pumped (by a DC or AC current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide.
Additionally, semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation is output perpendicular to the wafer surface. One type of SEL is the vertical cavity surface emitting laser (VCSEL). The VCSEL structure usually consists of an active (gain) region sandwiched between two distributed Bragg reflector (DBR, or mirror stack) mirrors. The DBR mirrors of a typical VCSEL can be constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections). Other types of VCSELs sandwich the active region between metal mirrors. The area between the reflective planes is often referred to as the resonator, or resonance cavity.
Semiconductor diode lasers are attractive as sources of optical energy in industrial and scientific applications. For example, semiconductor diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Also, semiconductor diode lasers are monolithic devices that do not require combining a resonance cavity with external mirrors and other structures to generate a coherent output laser beam. Further, the continuous development of semiconductor lasers in the last two decades has significantly improved their maximum output power to the kilowatt range, spanning wavelengths of more than 10 xcexcm. Semiconductor lasers are now widely used in industrial processing, telecommunications, data storage, and the like. Despite these improvements, however, semiconductor diode lasers still have a relatively low power output, as compared to other, conventional types of laser devices.
Semiconductor diode lasers, including quantum well lasers, are bipolar semiconductor laser devices. A diode laser typically has n-type layers on one side, and p-type layers on the other side, of an undoped active or core region. Such bipolar laser devices rely on transitions between energy bands in which conduction band electrons and valence band holes, injected into the active region through a forward-biased p-n junction, radiatively recombine across the bandgap. Thus, in diode lasers, the bandgap of the available active region materials essentially determines, and limits, the lasing wavelength. For example, the longer the laser wavelength needed, the smaller the required material bandgap, and vice versa. Unfortunately, the characteristics of small bandgap materials can make it difficult, expensive, or impractical to obtain lasing operation at certain desired wavelengths, such as mid-infrared (mid-IR or MIR) wavelengths.
Semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced. Some of those photons travel in a direction perpendicular to the reflective planes of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times. When those photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If most electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflective planes tends to increase. A typical laser includes a small difference in reflectivity between its mirrors. The primary laser output is emitted through the reflective plane having lower reflectivity.
The aforementioned QC was initially described in U.S. Pat. No. 5,457,709, which is incorporated herein by reference in its entirety. See also U.S. Pat. Nos. 5,509,025, 5,901,168, and U.S. Pat. No. 6,055,257, which are incorporated herein by reference in their entireties. Unlike diode lasers, QC lasers are unipolar, that is, they are based on one type of carrier (typically electrons in the conduction band), which make inter-subband transitions between energy levels created by quantum confinement. In a unipolar semiconductor laser, electronic transitions between conduction band states arise from size quantization in the active region heterostructure. The inter-subband transitions are between excited states of coupled quantum wells for which resonant tunneling is the pumping mechanism.
A single active region unipolar semiconductor laser is possible, but multiple active regions may be used as well. QC lasers, for example, typically comprise an active region having a plurality (e.g., 25) of essentially identical undoped active regions, sometimes referred to as radiative transition (RT) regions. Each active (RT) region comprises a plurality of semiconductor layers, and has quantum well regions interleaved with barrier regions, to provide two or more coupled quantum wells. These coupled quantum wells have at least second and third associated energy states for the charge carriers (e.g. electrons). The second energy state is of lower energy than the third energy state, which correspond to second and third wavefunctions, respectively. The energy difference between the third and the second energy states determines the laser emission wavelength. The energy difference between second and third energy states is in turn determined by the arrangement of all the coupled quantum wells in the active region. The arrangement includes the number of quantum wells, the thickness of each individual quantum well, and the energy height and thickness of each energy barrier layer between two neighboring quantum wells.
A multilayer carrier injector or injection region, sometimes referred to as an xe2x80x9cinjection/relaxationxe2x80x9d (I/R) or xe2x80x9cenergy relaxationxe2x80x9d region, is disposed between any two adjacent active regions. Thus, a given active region is separated from an adjoining one by an I/R region. The I/R region, like the active region, also typically comprises a plurality of semiconductor layers. Each active region-I/R region pair (i.e., each RT-I/R pair) may also be referred to as a xe2x80x9crepeat unit.xe2x80x9d At least some of the layers in each I/R region are doped, and in any case, the I/R regions as well as the active regions are unipolar. The aforementioned U.S. Pat. No. 5,457,709 discloses a technique for designing a QC laser that uses the inter-subband transition between energy levels of a coupled quantum well structure and an I/R region with a digitally graded energy gap structure, and the nominal structure of a QC laser is described in the aforementioned U.S. Pat. No. 5,509,025 . Unlike a diode laser, the layers of the multilayer semiconductor QC laser structure are either undoped, or, if doped, they are of the same type, e.g. n-type.
An operating voltage is provided across the multilayer semiconductor structure. This causes substantial energy relaxation of charge carriers (e.g. electrons) in the I/R regions, some of which are introduced into the I/R region from an adjacent active region. These carriers undergo a radiative transition, leading to lasing. QC lasers and various improvements to QC lasers since their initial description in the aforementioned U.S. Pat. No. 5,457,709 are discussed in Faist, J. et al., xe2x80x9cHigh Power Mid-infrared (xcex0 about 5 xcexcm) Quantum Cascade Lasers Operating Above Room Temperature,xe2x80x9d Appl. Phys. Lett., vol. 68, pp. 3680-3682 (1996) and Sirtori, C. et al., Appl. Phys. Lett., vol. 68, p. 1745 (1996).
For several types of applications, especially in the area of optical sensors for atmospheric trace gases, it is advantageous to operate with single mode, single frequency lasers. The use of distributed feedback (DFB) QC lasers for this purpose has been extensively explored, as described in Faist, J. et al., xe2x80x9cDistributed Feedback Quantum Cascade Lasersxe2x80x9d, Appl. Phys. Lett., vol. 70, No. 20, pp. 2670-2672 (1997); Gmachl, C., et al., IEEE Photonics Technol. Lett., vol. 9, p. 1090 (1997); Gmachl, C., et al., Appl. Phys. Lett., vol. 72, p. 1430 (1998).
Unlike other semiconductor lasers, such as diode lasers, the lasing wavelength of a QC laser is essentially determined by quantum confinement, i.e., by the thickness of the layers of the active regions, rather than by the bandgap of the active region material. The lasing wavelength thus can be tailored over a wider range than a diode laser, using the same semiconductor material. For example, QC lasers with InAlAs/InGaAs active regions have been tailored to operate at mid-IR wavelengths in the 3.5 to 13 xcexcm range.
In addition, diffraction gratings may be used to further control the operation frequency of semiconductor lasers. Examples of this type of laser are DFB lasers, lasers employing DBRs, and grating coupled surface emitting lasers (GCSELs). GCSELs and related technology are described in Lowery, A. J., xe2x80x9cPerformance Comparison of Gain-Coupled and Index-Coupled DFB Semiconductor Lasers,xe2x80x9d IEEE J. Quantuin Electronics, vol. 30, no. 9, pp. 2051-2063 (1994); Kock, A., xe2x80x9cSingle-mode and Single-beam Emission from Surface Emitting Laser Diodes Based on Surface-mode Emission,xe2x80x9d Appl. Phys. Lett., vol. 69 (24), pp. 3638-3640 (1996); Rast, A., et al., xe2x80x9cNew Complex-Coupled DFB-Laser with a Contacted Surface Grating for xcex=1.55 xcexcmxe2x80x9d, IEEE Proceedings Optoelectronics, vol. 142, no. 3, pp. 162-164 (1995).
When using a diffraction grating, both the thickness of the active region layers and the diffraction grating determine the lasing wavelength, as follows. In a QC laser, the characteristics of the active region (e.g., the number and layer thicknesses of coupled quantum wells) can be varied to determine the laser emission wavelength range. The diffraction grating is used to precisely pick out a much narrower wavelength range within that determined by the active region layer thickness. The grating controls the lasing wavelength more precisely than the lasing wavelength range determined by the layer thickness alone. However, the lasing wavelength selected by the diffraction grating cannot exceed the available laser wavelength range determined by the layer thickness and the whole laser structure.
Thus, a unipolar injection laser, such as a QC laser, offers several advantages over bipolar semiconductor lasers. Compared to bipolar semiconductor lasers, these QC lasers have a frequency response not limited by electron/hole recombination, a narrow emission linewidth because the line-width enhancement factor is (theoretically) zero, and a weaker temperature dependence of the lasing threshold. Additionally, as noted above, appropriately designed QC semiconductor lasers can have an emission wavelength in the spectral region from the mid-IR to the submillimeter region, which is entirely determined by quantum confinement.
An alternative approach to fabricate a high-power, single-mode laser is to use a so-called xe2x80x9ccurved grating.xe2x80x9d In such a laser, with a mode made up of counter-propagating diverging beams, the rulings of the diffraction gratings are curved to reflect one traveling wave into the other. See, e.g., Lang, R. J., xe2x80x9cDesign of Aberration-Corrected Curved Mirror and Curved-Grating Unstable-Resonator Diode Lasers,xe2x80x9d IEEE J. Quantun Electron., vol. 30, p. 31 (1994). A method for fabricating the grating for a DFB semiconductor laser is disclosed in xe2x80x9cSurface-Emitting Distributed Feedback Semiconductor Laser,xe2x80x9d S. Macomber et al., Appl. Phys. Lett. 51(7) pp. 472-474 (August 1987). This paper describes a technique in which a gold coating is deposited on a grating etched into the p-side of a semiconductor laser. The gold coating also serves as the p-contact for the laser. However, this approach employs an edge-emitting laser structure and has a large beam divergence angle (sometimes referred to as a diffraction angle) along the direction that is perpendicular to the laser surface, as well as a much higher optical power density at the laser facet that it is susceptible to catastrophic mirror damage. An edge-emitting semiconductor laser also typically has a an elliptical, as opposed to circular, laser beam cross-section. This can require correction and collimating, which can be expensive or otherwise impracticable or undesirable. Also, due to the nature of the curved grating, the output laser beam has a spatial phase difference distribution, which reduces optical beam quality.
Many applications also require lasers that operate in the mid-IR spectral range, e.g., between about 3 and 13 xcexcm. Such applications include remote chemical sensing, pollution monitoring, LIDAR (Laser Infrared Detection and Ranging), infrared counter-measure, and molecular spectroscopy. Unfortunately, there are few convenient laser sources that operate in the mid-IR spectral region. As noted above, for example, bipolar semiconductor diode lasers, including quantum well lasers, have too large a bandgap, making it difficult, if not impossible, to obtain lasing operation at mid-IR wavelengths. Some semiconductor lasers can operate in this wavelength range, but they require special cooling to a very low temperature, which can be costly.
QC lasers, however, as noted above, do not suffer these drawbacks, and can be designed to emit radiation at substantially any desired wavelength in a rather wide spectral region, including emissions in the mid-IR range. Therefore, QC lasers are desirable for such mid-IR range applications. For example, QC lasers may be employed advantageously as radiation sources for absorption spectroscopy of gases and pollutants, because at least some QC lasers can operate in the relevant wavelength region at or near room temperature and with relatively high output power. See, for instance, Faist, J. et al., Applied Physics Lett., Vol. 68, pp. 3680-3682 (1996); and C. Sirtori et al., xe2x80x9cMid-infrared (8.5 xcexcm) Semiconductor Lasers Operating at Room Temperature,xe2x80x9d IEEE Photonic Technol. Lett., vol. 9 (3), pp. 294-296 (1997), both incorporated herein by reference.
Some applications require high laser output power, especially single-mode lasers. For example, for LIDAR, differential absorption LIDAR (DIAL), and other remote chemical sensing systems, a spatially coherent, single-mode, high-power laser beam in the appropriate wavelength range can greatly increase the sensing range. Single-mode emissions from QC lasers can be achieved by incorporating a DFB or DBR grating into the laser structures.
It is difficult, however, to achieve a spatially coherent, single-mode, high-power laser beam using conventional edge-emitting QC laser structures as the source of mid-IR radiation. To obtain high output power with an edge-emitting QC laser, one has to use either a very high injection current density into a narrow stripe laser or use a broad area laser to increase the lasing area. A high injection current density will cause severe device heating and thus significantly limit the maximum output laser power and reduce the laser lifetime.
With a broad area laser, on the other hand, it is very difficult to maintain single-mode operation under high current injection operation because of self-induced filamentation problems, which causes multi-mode lasing. See, e.g., G. P. Agrawal and N. K. Dutta, Semiconductor Lasers (2nd edition; New York: Van Nostrand Reinhold, 1993). The self-induced filamentation effect produces a multi-spatial mode laser beam, which diminishes the laser""s power and performance. The multi-mode laser beam is very difficult to focus, which is especially problematic when a long propagation distance or a very small focus beam size is required. The multi-mode laser beam is less coherent and, thus, very difficult to use in some applications. In addition, as noted above, the output beam of an edge-emitting laser always has a very large divergence angle in the direction perpendicular to the laser top surface.
Surface-emitting QC lasers, by contrast, show great promise for applications requiring high output power. A surface-emitting DFB semiconductor laser has the potential to produce higher single-mode output power than an edge-emitting laser, because a larger lasing area can be used and the internal loss of the laser structure can be reduced. Under current technology, the output from surface emitting lasers can be spatially coherent if the width, or the lateral dimension, of the lasing region is limited to about 5 xcexcm. To obtain higher output power, however, it is advantageous to provide a lasing region with a width of 50 xcexcm or more. Unfortunately, simply increasing the width will lead to spatially incoherent operation at high current injection. Thus, there is a need for techniques for fabricating a surface emitting QC laser with both a wide lasing region and a spatially coherent output beam.
Several approaches have been proposed to prevent the filamentation problem in a broad-area semiconductor laser. The typical solution is to create a so-called unstable resonance cavity (also referred to as an unstable resonant cavity or unstable resonator) within the laser device. There are several ways to create this kind of cavity for a high power laser. A semiconductor laser with a continuous unstable resonator has been described in Guel-Sandoval, S. et al., xe2x80x9cNovel High-Power and Coherent Semiconductor Laser with a Shaped Unstable Resonator,xe2x80x9d Appl. Phys. Lett., vol. 66 (1995): pp. 2048-2050, which is incorporated herein by reference in its entirety. This paper describes a means for inducing a quadratically varying index of refraction across the lateral dimension of a wide-stripe semiconductor laser, to cause beam divergence and coherent operation in the laser. Another approach, using a curved grating, is described in the Lang reference, described above.
A new way to fabricate a grating-coupled surface-emitting diode laser with an unstable resonance cavity is disclosed in aforementioned U.S. Pat. No. 5,727,016. This patent describes the use of a variable index refraction layer, having an approximately parabolic-shaped trough therein. The refraction layer is positioned adjacent to the active lasing region. A straight-toothed, second-order diffraction grating contacts the refraction layer to produce a broad, spatially coherent output beam. Kastalsky, A., xe2x80x9cInfrared Intraband Laser Induced in a Multiple-Quantum-Well Interband Laser,xe2x80x9d IEEE J. Quantum Electronics, vol. 29, no. 4, pp. 1112-1115 (1993).
Therefore, there is a need for improved surface-emitting QC lasers, which produce high-power, spatially coherent, single-mode output beams, having a small divergence angle. Such devices especially would be useful, for example, for remote chemical sensing and LIDAR applications. A compact, low-cost and reliable high-power, spatially coherent, single-mode, mid-IR semiconductor laser can greatly reduce the system cost and reliability for these applications.